Conventional constant-speed running control devices for vehicles are of the constitution as illustrated in FIG. 1(a). This drawing gives a block diagram showing this type of prior-art constant-speed running control device for vehicles disclosed in for example Laid-Open Japanese Pat. No. Sho 58-39311. In this drawing, 1 indicates a SET switch which enables a driver to set the start of constant-speed running; 2 is a CANCEL switch which is operated by operation of a brake system (not illustrated), allowing cancellation of constant-speed running; and 3 is a vehicle speed sensor for sensing running speed of the vehicle. It, comprising a rotary body 3a which has four magnetic poles and is driven by means of a meter cable (not illustrated) for transmitting rotation of a transmission (not illustrated) and a reed switch 3b, outputs a pulse-train signal at a frequency proportional to the running speed.
Turning on the main switch 5, which functions as a power switch for supplying electric power from the battery 4 for vehicle, provides the electric power to the control device 6, which will then operate. The control device 6 includes an operational processing circuit 6a of a microcomputer, for example, therewithin. It inputs signals from the SET switch 1, the CANCEL switch 2 and the vehicle-speed sensor 3, and outputs various control signals, after performing various processing operations, to make automatic control to keep a vehicle running speed v.sub.s at a target speed v.sub.r.
A throttle actuator 7 is a motor-driven throttle actuator disposed in an intake passage 8 of an engine (not illustrated). Receiving various signals from the control device 6, it opens and closes a throttle valve 9 in interlock with an accelerator pedal (not illustrated).
This throttle actuator 7 drives the throttle valve 9 through a wire 7b connected to a link 7a which is turned by a motor (not illustrated).
The angle of rotation of the link 7a corresponding to the opening of this throttle valve 9 is sensed by a built-in potentiometer (not illustrated), and given as a throttle position signal to the control device 6.
The link 7a and the motor are connected by means of a magnetic clutch (not illustrated); the connected state is controlled by a magnetic clutch signal from the control device 6.
Next the operation of a conventional constant-speed running control device of the above-mentioned constitution for vehicles is as follows. First, when the main switch is turned on by a driver, the power is supplied from the battery 4 to start the operation of the control device 6, thus processing the output of the vehicle-speed sensor 3. This vehicle-speed sensor 3 outputs a pulse-train signal having a frequency proportional to a running speed v.sub.s at which the vehicle is traveling. The control device 6 measures the period of this pulse, thereby determining the running speed v.sub.s.
When the driver operates the SET switch 1, a signal from this switch is given to the control device 6, and the running speed v.sub.s at this time is stored as the target speed v.sub.r ; thus the control of constant-speed running is started.
Henceforth, the control device 6 compares an actual running speed v.sub.s changing every second, with the target speed v.sub.r, and outputs a control signal to drive the throttle actuator 7 and control the opening of the throttle valve 9 so that the vehicle can travel at the target speed v.sub.r.
Namely, when the actual running speed v.sub.s is slower than the target speed v.sub.r, the control device 6 will output a THROTTLE OPEN CONTROL DRIVE signal to open the throttle valve 9 to a specific amount of opening, and reversely when the actual running speed v.sub.s is higher, a THROTTLE CLOSE CONTROL DRIVE signal will be outputted to close the throttle valve 9 to a specific amount of opening, and therefore the vehicle can travel at a constant speed without the driver's actuation of the accelerator pedal.
If the driver operates the brake system in mid course of such constant-speed running control, the CANCEL switch 2 will operate to give a CONSTANT-SPEED RUNNING CANCEL signal to the control device 6.
Upon receiving this signal, the control device 6 outputs a signal to release the magnetic clutch and the throttle actuator 7, receiving this signal, will release the magnetic clutch.
Therefore, henceforth the driver adjusts the opening of the throttle valve 9 by the accelerator pedal, thus controlling the running speed of vehicle.
In the conventional constant-speed running control device for vehicles that is constituted as described above, when the driver has turned on the SET switch 1 to run the vehicle at a fixed vehicle speed, the control device 6 first outputs a drive signal to control the opening of the throttle to a specific amount (setup amount). Thereafter it calculates every moment a controlled variable (trimmed amount) from a speed deviation .epsilon. between the target speed v.sub.r and the running speed v.sub.s and the acceleration .alpha., and outputs.
This controlled variable (trimmed amount) is equivalent to the gain of the control system, and is determined by the convergence and speed deviation of the running speed v.sub.s with respect to the target speed v.sub.r.
The trimmed amount, if much, provides good convergence, but tends to cause a hunting running speed, resulting in lowered driving comfort. Reversely, the trimmed amount, if little, tends to reduce the convergence.
In the meantime, the first specific amount (setup amount) has a purpose to control in advance the throttle opening to the vicinity of an opening required, in order to maintain the target speed.
This setup amount takes part in running safety thereafter; in case of an improper setup amount, the speed deviation immediately after setting will increase.
Formerly, this setup amount was a fixed value, or a specific value proportional to the running speed and target speed.
This is the amount determined when the vehicle has a standard throttle actuator function, and on the assumption that the road is level.
The setup amount required, however, varies with a change in the amount of mechanical play of the throttle actuator.
The play will be explained by reference to FIG. 1(b). In FIG. 1(b), there exists a play between a motor 11 and a link 7a, between the link 7a and a wire 7b, with the wire 7b itself, between the wire 7b and a throttle link 12b, and between the throttle link 12a and 12b. Furthermore, a period of time from the start of rotation of the motor 11 till the start of movement of the throttle valve 9 in response to this rotation is also called "play".
This play is likely to vary with vehicles or with age. If there exists too much play, throttle opening is insufficient with the specific setup amount. Reversely, if the play is too little, the throttle opening will be controlled in excess with the specific setup amount.
Furthermore, there is such a drawback that the play varies with road condition; when the driver turns on the SET switch 1 on an upward hill for example, the throttle opening will become insufficient in the case of the specific setup amount, and, reversely, will become excessive on a downward hill, resulting in an increased speed deviation immediately after setting.
This speed deviation is gradually converged by the variable (trimmed amount) to be outputted after the completion of output of the setup amount; however, since the trimmed amount is determined by convergence and trade-off of speed deviation; there is such a shortcoming that if the setup amount is improper, the speed deviation increases.
Furthermore, there is a drawback that, particularly when the vehicle is traveling with a specific setup amount on a downward hill or with a little amount of play, the set deviation will temporarily become positive, that is, the running speed will exceed the target speed, which, being a vehicle behavior immediately after setting, will cause the driver to have a feeling of uneasiness.
The present invention has been accomplished in an attempt to solve these problems, and has as its object the provision of a constant-speed running control device for vehicles that is capable of reducing an influence of road conditions and the amount of play, controlling to rapidly bring the running speed close to the target speed, and, especially, preventing the speed deviation from becoming positive at the time of setting.